Oxidizers are commonly used to effectively destroy organic and inorganic contaminants. Some of the typical applications of oxidizers include treatment of water systems and inactivation of bacteria and viruses in various media.
Although oxidizers are used in numerous applications, there are also applications where they are not used even though their utility is well established. The reason these oxidizers are not used often relates to their instability during storage. Oxidizers such as hypochlorous acid, peracids, and chlorine dioxide, for example, could be used in more applications than the disinfection applications that they are already used in if their stability can be improved. The problem with some of these powerful oxidizers such as hypochlorous acid and peracids is that their activity level tends to decrease during storage. Since the effectiveness of the oxidizers in various applications depends on their concentrations, activity levels, and the level of demand on the oxidizer as measured by its oxidation reduction potential (ORP), a reduction in the activity level of the oxidizers impedes their performance in the various applications. Thus, even if an oxidizer is initially highly effective, the effectiveness decreases during storage.
A few methods are currently used to get around this storage problem. One of these methods, which is the point-of-use generation method or the in-situ method, is desirable because it eliminates the need for prolonged storage. However, on a practical level, these point-of-use generation methods are not widely employed because they require expensive equipment and specialized expertise. Other in-situ generation methods involve adding the reagents to the water to produce the target product. However, when doing this, significant dilution of reagents as well as competing reactions impede the level of conversion to the target product.
Sometimes, the reagents are coated to provide a protective shield or barrier between the reagents and the environmental elements, thereby making the reagents easier to store and use in formulations. The protective coatings are designed so that when they are combined with water, they dissolve and rapidly release the reagents. Silicates, for example, are widely used in laundry detergent applications. In the alkaline condition induced by the laundry formulation, the silicate coating rapidly dissociates and releases the encased additives into the bulk water. There are also instances where a highly hydrophobic coating such as a wax or slow-dissolving coating is used for time-release purposes. These cases operate on the basis of a mechanism similar to the mechanism of the silicate coating in that the outer coating material quickly dissolves to expose the enclosed material to the solvent in the environment.
Various compositions have been made to enhance the bleaching/oxidizing performance in an application. Such enhancement is desirable because the generally effective hydrogen peroxide donors such as percarbonate, perborate, and persulfate-based additives do not remove stubborn stains from clothing. To enhance their bleaching ability under the conditions that are typical to the application (e.g., laundry water), precursors are added to induce formation of a more effective bleaching agents (e.g., tetraacetyl-ethylenediamine (TAED)) in-situ. However, this addition of bleaching agent precursors has its disadvantages. For example, high concentrations of additives are needed to achieve effective results, increasing both the cost and inconvenience.
Another way of enhancing peroxygen compounds' performance is to make them more stable, thus allowing long-term storage. Sometimes, the peroxygen compounds and the formulations they are used in are coated to enhance storage stability. These coatings, however, do not always dissolve quickly and therefore increase the time it takes for the peroxygen compound to become effective. One of the ways to allow long-term storage of oxidizers such as potassium monopersulfate and chlorine is to store them in packages or bags. The packages or bags are designed to dissolve in water, so that they can be directly thrown into a body of water. Although the use of bags provides for easy application in large scale or macro applications, their utility is limited in that they can be used only for applications of a certain scale.
U.S. Pat. No. 6,699,404 to Speronello (“the Speronello patent”) discloses a massive body having a porous structure which substantially increases the percent conversion of chlorite to chlorine dioxide when compared to chlorite powder. The Speronello patent discloses two types of massive bodies: a water soluble type and a substantially water insoluble type. The substantially water insoluble massive body forms a porous framework that provides a higher efficiency of the conversion compared to the water-soluble massive body. According to the test data provided in the Speronello patent the maximum concentration of chlorine dioxide produced by a massive body that forms the porous framework is 149.4 mg/L. The water-soluble massive body reported (example 4) a maximum 27.4 mg/L.
In order to achieve 70% or more conversion of the chlorite to chlorine dioxide using the method disclosed in the Speronello patent, a substantial amount of inert materials are added to produce the porous structure or the porous framework. The level of inert salts ranges from 18 wt. % to 80 wt. %, with higher weight percentages increasing the conversion efficiency. The high levels of inert material, particularly in the water-soluble massive body, are further illustrated in commercial practice. For example, Aseptrol®, which is the commercialized product embodying the invention disclosed in the Speronello patent, is a water soluble tablet that requires 1.5 grams of Aseptrol® to 1 liter of water to produce 100 mg/L chlorine dioxide. This equates to approximately 67 mg/L chlorine dioxide based on 1 gram tablet per liter. The weight-% yield, which is defined as weight chlorine dioxide per weight of tablet, is low because of the high level of inert material. According to the data reported in the Speronello patent, the weight % yield is less than 15 wt. %, and less than 3% in the case of the water-soluble massive body. Based on the commercial product Aseptrol®, the weight percent yield of the water soluble commercial product is 6.7 wt. %.
It is desirable to increase the concentration of chlorine dioxide produced by a given mass of tablet to improve the economics based on the cost per pound of the tablet material versus pounds of chlorine dioxide produced. Such increase would also result in an overall performance enhancement offered by higher concentrations of chlorine dioxide. To achieve this objective, tablet conversion efficiency of >70% and a high reactant weight percent are desirable. It is also desirable to substantially increase the concentration of chlorine dioxide using a completely water-soluble composition to eliminate the problems associated with water insoluble constituents or byproducts such as residue silica based clays, or mineral salts such as calcium sulfate.
U.S. Pat. Nos. 6,384,006 and 6,319,888 to Wei et al. (“the Wei patents”) disclose a system for forming and releasing an aqueous peracid solution. The system includes a container and a peracid forming composition that includes about 10-60 wt. % of a chemical heater that, upon contact with water, generates heat to increase the yield of the peracid.
The Wei patents describe the potential use of a viscosity modifier within a permeable container to increase the viscosity in the localized area from about 300 to about 2,000 centipoise. The increased viscosity restricts and slows down the movement of peracid precursor and/or peroxygen source out of the permeable container. This results in an increased residence time of the peracid precursor and peroxygen source within the permeable container, which in turn translates to a greater reaction rate.
U.S. Pat. No. 6,569,353 to Giletto et al. (“the Giletto patent”) discloses using silica gel to increase the viscosity of various oxidants including an in-situ generated oxidant in order to keep them in intimate contact with the agents targeted for oxidation.
U.S. Published Application No. 2001/0012504 to Thangaraj et al. (“the Thangaraj application”) discloses a composition for producing chlorine dioxide comprising an acid source and a chlorite source, and a method comprising enclosing the composition in a gelatin capsule or membrane sheet such as a “tea bag”.
In order to improve reaction kinetics, the above references teach using substantial quantities of inert materials to either provide a porous structure as in the case of the Speronello patent, or heat as in the cases of the Wei patents. While viscosity modifiers are referenced in the Wei patents, the viscosity range disclosed in the Wei patents does not reflect the formation of a gel.
Search still continues for a method of stabilizing reactive components for storage without compromising or limiting their function during usage.